Knee prosthesis with rotatable post

ABSTRACT

A knee joint prosthesis for implantation to replace a portion of a knee joint. The knee joint prosthesis includes a tibial tray and a bearing member that engages the tibial tray. The prosthesis also includes an insert with a base that is fixed to the bearing member and an extension portion that extends away from the bearing member and the tibial tray. The extension portion has an axis, and the extension portion includes a threaded portion. The prosthesis further includes a post that is rotatably coupled to the extension portion, and the post is rotatable about the axis of the extension portion relative to the insert. Furthermore, the prosthesis includes a retention member that threadably couples to the threaded portion of the insert to limit movement of the post along the axis away from the bearing member and the tibial tray.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/179,148 filed on Jun. 5, 2009. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The following relates to a knee prosthesis and, more particularly, to aknee prosthesis with a rotatable post.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

The human anatomy includes many articulating joints. For example, thefemur and tibia cooperate to form a knee joint of the human anatomy andarticulate to allow ease of walking and mobility. Nevertheless, overtime, disease and injury may deteriorate the knee joint, such thatarticulation of the joint becomes painful or impractical. When suchdeformities or injuries occur, anatomical replacements, particularlyimplants and prosthetics, can be placed in the femur or the tibia, orboth to replace the damaged portions and restore the naturalarticulation of the knee joint.

However, replacing only the articulating portions is not alwayspractical or possible. Particularly, certain tissues such as theanterior cruciate ligament, the posterior cruciate ligament, tendons,and muscles may not be able to withstand natural loading of the joint,even if the articulating portions of the joint are replaced byprosthetic members. Thus, the prosthetic knee may need to also replaceand/or compensate for those tissues.

One exemplary component of this type of prosthetic knee is a posteriorstabilized prosthetic knee joint. The posterior stabilized kneeprosthetic may include a post, particularly a posterior stabilized (PS)post, which extends superiorly from a tibial component to operablyengage a femoral component, or the anatomical femur, to constrainposterior movement of the knee, which is not otherwise able to beconstrained by the anatomical soft tissues. Improvements of this type ofprosthetic knee are desired.

SUMMARY

A knee joint prosthesis for implantation to replace a portion of a kneejoint is disclosed. The knee joint prosthesis includes a tibial tray anda bearing member that engages the tibial tray. The prosthesis alsoincludes an insert with a base that is fixed to the bearing member andan extension portion that extends away from the bearing member and thetibial tray. The extension portion has an axis, and the extensionportion includes a threaded portion. The prosthesis further includes apost that is rotatably coupled to the extension portion, and the post isrotatable about the axis of the extension portion relative to theinsert. Furthermore, the prosthesis includes a retention member thatthreadably couples to the threaded portion of the insert to limitmovement of the post along the axis away from the bearing member and thetibial tray.

In another aspect, a knee joint prosthesis for implantation to replace aportion of a knee joint is disclosed. The knee joint prosthesis includesa tibial tray and bearing member that engages the tibial tray. Thebearing member includes a tray engaging surface that engages the tibialtray. The knee joint prosthesis additionally includes a post and aninsert with a base that is fixed to the bearing member and an extensionportion that extends away from the bearing member and the tibial tray.The post is rotatably coupled to the extension portion to be rotatableabout an axis of the extension portion, and the post is limited againstmovement along the axis away from the bearing member and the tibialtray. The base includes an inferior terminal end that is disposedsuperior to the tray engaging surface of the bearing member.

Additionally, a knee joint prosthesis for implantation to replace aportion of the knee joint is disclosed. The knee joint prosthesisincludes a tibial component having a tibial tray with a bearing engagingsurface and a bearing member that is fixed to the tibial tray to bedisposed adjacent the bearing engaging surface of the tibial tray. Thebearing member also includes a tray engaging surface that engages thetibial tray. Furthermore, the tibial component includes an insert with abase that is embedded within the bearing member to be fixed to thebearing member. The base includes an inferior terminal end that isdisposed superior to the tray engaging surface of the bearing member.The insert further includes an extension portion that extends away fromthe bearing member and the tibial tray. The extension portion has anaxis, and the extension portion includes a threaded portion.Furthermore, the tibial component includes a post having a bore andcounterbore that receive the extension portion to rotatably couple thepost to the extension portion for rotation about the axis relative tothe insert. Additionally, the tibial component includes a washer that isdisposed between the post and the bearing member, and the washerinhibits wear due to rotation of the post relative to the bearingmember. Also, the tibial component includes a nut that receives theextension portion to threadably couple to the threaded portion of theextension portion and to limit movement of the post along the axis awayfrom the bearing member and the tibial tray. The nut is received withinthe counterbore of the post. Moreover, the tibial component includes aweldment that bonds the retention member to the insert. Still further,the knee joint prosthesis includes a femoral component with an inferiorportion having an opening. The inferior portion is coupled forarticulation on the bearing member, and the post is moveably receivedwithin the opening.

Moreover, a method of assembling a knee joint prosthesis for replacing aportion of a knee joint is disclosed. The method includes engaging abearing member with a tibial tray. The bearing member has an insertinsert-molded therein with a base embedded within the bearing member. Aninferior terminal end of the insert is disposed superior to a trayengaging surface of the bearing member, and an extension portion of theinsert extends away from the bearing member. The method further includesrotatably coupling a post to the extension portion for rotation of thepost about an axis of the extension portion. Additionally, the methodincludes threadably coupling a retention member to the extension portionto limit movement of the post along the axis away from the bearingmember. Still further, the method includes coupling a femoral componentfor articulation on the bearing member, wherein the post is moveablyreceived within an opening of the femoral component.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a perspective view of an exemplary embodiment of a knee jointprosthesis according various teachings of the present disclosure;

FIG. 2 is a perspective view of a tibial component of the knee jointprosthesis of FIG. 1;

FIGS. 3A and 3B are perspective views of the tibial component of theknee joint prosthesis of FIG. 1;

FIG. 4 is a perspective view of a bearing member of the knee jointprosthesis of FIG. 1;

FIG. 5 is a perspective view of a post of the knee joint prosthesis ofFIG. 1;

FIG. 6 is a perspective view of an insert of the knee joint prosthesisof FIG. 1;

FIG. 7 is a perspective view of a retention member of the knee jointprosthesis of FIG. 1;

FIG. 8 is a perspective view of the insert and the retention member ofthe knee joint prosthesis of FIG. 1;

FIG. 9 is a perspective view of a washer of the knee joint prosthesis ofFIG. 1;

FIGS. 10A and 10B are alternative plan views of the tibial component ofthe knee joint prosthesis; and

FIG. 11 is a cross sectional view of the tibial component of the kneejoint prosthesis of FIG. 1.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

Referring initially to FIG. 1, a knee joint prosthesis 10 according tovarious exemplary embodiments is illustrated. The knee joint prosthesis10 can be implanted within a patient to replace at least a portion of aknee joint. Generally, the knee joint prosthesis 10 can include afemoral component 12 and a tibial component 14. The femoral and tibialcomponents 12, 14 are coupled for articulation, as will be discussed ingreater detail below, to emulate an anatomical knee joint.

The knee joint prosthesis 10 can incorporate one or more features of thecommercially available VANGUARD® SSK Revision System of Biomet, Inc.,located in Warsaw, Ind. In some embodiments, the knee joint prosthesis10 can be a posterior stabilized knee prosthetic; however, it will beappreciated that the prosthesis 10 can be of any suitable type withoutdeparting from the scope of the present disclosure. Furthermore, it willbe appreciated that the knee joint prosthesis 10 can include only thetibial component 14 coupled to an anatomical femur (not shown).

As shown in FIG. 1, the femoral component 12 can include a stem 16. Thestem 16 can be axially straight and generally cylindrical to be receivedwithin and fixed to a resected femur (not shown). It will be appreciatedthat the stem 16 can be optionally included within the prosthesis 10.For instance, the femoral component 12 can be adapted for a fullyconstrained knee joint prosthetic that includes the stem 16. The femoralcomponent 12 can also be adapted for a posterior stabilized knee jointprosthetic that does not include the stem 16.

Furthermore, the femoral component 12 can include an inferior portion18, which is fixed to the stem 16. The inferior portion 18 can include afirst condyle 20 and a second condyle 22. Furthermore, the inferiorportion 18 can include an opening 24 (i.e., an inter-condylar box),which is defined between the first and second condyles 20, 22. It willbe appreciated that the inferior portion 18 could include only the firstcondyle 20 or the second condyle 22 with remaining portions of the femurbeing anatomical bone.

The tibial component 14 can include a tibial tray 26 having a tibialstem 27. The tibial stem 27 can be elongate and frusto-conical in shapeand can be received within and affixed to a resected tibia bone (notshown). The tibial component 14 can further include a bearing member 28that engages the tibial tray 26 to be disposed adjacent and supported bythe tibial tray 26. In addition, the tibial component 14 can include aninsert 30 (FIGS. 2, 6, and 8) that is partially embedded and fixedagainst rotation within the bearing member 28. The tibial component 14can also include a post 32 (FIGS. 1 and 2) that is rotatably coupled tothe insert 30. Furthermore, the tibial component 14 can include a washer34 (FIGS. 2 and 9) that is disposed between the bearing member 28 andthe post 32. The tibial component 14 can include a retention member 36(FIGS. 2, 3A, 3B, 7, and 8) that is fixed to the insert 30 and thatlimits movement of the post 32 in a direction generally away (i.e.,superiorly) from the bearing member 28 and the tibial tray 26, as willbe discussed in greater detail below. As will be discussed, the tibialcomponent 14 facilitates assembly of the knee joint prosthesis 10, andthe tibial component 14 includes various features that make the kneejoint prosthesis 10 more robust.

As shown in FIG. 1, the tibial tray 26 can include a superior surface 38(i.e., a bearing engaging surface) that is substantially flat. Thetibial tray 26 can also include an inferior surface 40 that is oppositeto the superior surface 38. The stem 27 can be fixed to and can extendinferiorly from the inferior surface 40. It will be appreciated that theinferior surface 40 can abut and can be supported by a resected tibia(not shown). Furthermore, the tibial tray 26 can include one or moreanterior projections 42 that extend superiorly from the superior surface38. The tibial tray 26 can also include one or more posteriorprojections 41 (FIGS. 1 and 11) that extend superiorly from the superiorsurface 38. The tibial tray 26 can be made out of any suitable material,such as a metal (e.g., cobalt, cobalt alloy, etc). It will beappreciated that the tibial tray 26 can be suitable for accommodating acruciate retaining (CR) prosthetic, a posterior stabilized (PS)prosthetic, or a fully constrained (FC) prosthetic. Accordingly, thetibial tray 26 need not be adapted specifically for the knee jointprosthesis 10 of the present disclosure. Furthermore, it will beappreciated that the tibial tray 26 can be of a type included in thecommercially available VANGUARD® 360 system or the REGENEREX® system ofBiomet, Inc. located in Warsaw, Ind.

Also, as shown in FIGS. 1, 2, 3A, 3B, and 4, the bearing member 28 caninclude an inferior surface 44 (i.e., a tray engaging surface) that issubstantially flat (FIGS. 1, 2-4, and 11). The inferior surface 44 canbe a surface located furthest in the inferior direction. The bearingmember 28 can also include one or more anterior projections 43 (FIGS. 1,3B, and 11) that extend in an anterior direction from the bearing member28.

The bearing member 28 can engage the tibial tray 26 in any suitablefashion. For instance, as shown in FIG. 11, the bearing member 28 can befixed to the tibial tray 26 such that the inferior surface 44 of thebearing member 28 engages, abuts, and is supported by the superiorsurface 38 of the tibial tray 26. Furthermore, the knee joint prosthesis10 can include an engagement member 45 (FIG. 1), which is elongate. Asshown in FIGS. 1 and 11, the engagement member 45 can be received withina recess 47 defined between the anterior projections 42, 43 of thetibial tray 26 and bearing member 28, respectively, and the engagementmember 45 can fixedly engage the bearing member 28 to the tibial tray26. Furthermore, the posterior projection 41 of the tibial tray 26 canhook onto and retain a superior posterior surface 39 to further fix thebearing member 28 to the tibial tray 26.

Moreover, the bearing member 28 can include a superior surface 46, whichis non-planar. The superior surface 46 can include a plurality ofrounded articulation recesses 48 a, 48 b. As shown in FIG. 1, thearticulation recesses 48 a, 48 b can receive the first and secondcondyles 20, 22 of the femoral component 12, respectively, such that thefirst and second condyles 20, 22 can articulate on the bearing member28. Also, a central recess 50 can be defined between the articulationrecesses 48 a, 48 b.

The bearing member 28 can be made out of any suitable material. Forinstance, the bearing member 28 can be made out of a polymer, such asultra-high molecular weight polyethylene (UHMWPE). As such, the bearingmember 28 can allow the femoral component 12 to articulate on thebearing member 28 with relatively low wear.

In addition, as shown in FIGS. 2, 6 and 8, the insert 30 can include abase 52, which is fixed to the bearing member 28. The insert 30 caninclude an extension portion 54, which extends away from and is exposedfrom the bearing member 28. The base 52 and the extension portion 54 ofthe insert 30 can be integrally attached so as to be monolithic;however, the base 52 and the extension portion 54 can be removablycoupled in some embodiments. The extension portion 54 can define an axisA. Moreover, the insert 30 can be made out of any suitable material,such as a metal (e.g., cobalt, cobalt alloy, etc.).

As best shown in FIGS. 6 and 8, the base 52 can be rounded anddisc-shaped. The base 52 can also be concentric and symmetrical aboutthe axis A of the extension portion 54. The base 52 can also include aninferior terminal end 55. The base 52 can be fixed to the bearing member28 in any suitable fashion. For instance, in some embodiments, theinsert 30 can be insert-molded to the bearing member 28. As such, theinsert 30 can be disposed in a mold (not shown) during injection moldingand formation of the bearing member 28. Also, the base 52 can include aplurality of through holes 56 spaced about the axis A. Each through hole56 can receive the molten material of the bearing member 28 during thisinjection molding process to further secure the base 52 to the bearingmember 28. Any air bubbles within the molten material can also bereceived within the through holes 56 to thereby enhance attachmentbetween the bearing member 28 and the base 52. Furthermore, the base 52can be surface treated to have an increased surface roughness (e.g.,grit blasted) to improve adhesion of the molten material to the base 52.

Also, as shown in FIG. 2, the base 52 can be fully embedded within thebearing member 28. For instance, in some exemplary embodiments, theinferior terminal end 55 of the base 52 can be disposed superior to theinferior surface 44 of the bearing member 28, and the base 52 can alsobe disposed inferior from the superior surface 46 of the bearing member28. Accordingly, the base 52 can be embedded and fixed to the bearingmember 28 in a very durable and robust fashion. Also, because the insert30 can be injection molded to the bearing member 28, manufacturing ofthe tibial component 14 can be facilitated.

Moreover, the extension portion 54 of the insert 30 can be substantiallycylindrical. An outer surface 58 of the extension portion 54 can beexposed from the bearing member 28. The extension portion 54 can includea threaded portion 60 on the outer surface 58 thereof. Also, theextension portion 54 can include a shoulder 59 that is disposed superiorto the threaded portion 60.

In addition, as shown in FIGS. 1, 2, 3A, 3B, and 5, the post 32 can besubstantially boxed-shaped and can include a superior portion 61 and aninferior portion 63. The superior portion 61 can extend slightly in aposterior direction of the knee joint prosthesis 10. The post 32 canalso include an anterior recess 62. Furthermore, the post 32 can includea bore 64. The bore 64 can be substantially cylindrical and can have awidth corresponding to the width of the extension portion 54 of theinsert 30. Moreover, the post 32 can include a counterbore 66. Thecounterbore 66 can be disposed on the superior portion 61 of the post32.

The bore 64 and the counterbore 66 can receive the extension portion 54so as to rotatably couple the post 32 to the extension portion 54 of theinsert 30. As such, the post 32 can rotate about the axis A of theextension portion 54. Accordingly, as will be discussed, the post 32 canrotate to compensate for any misalignment between the tibial component14 and the femoral component 12 of the prosthesis 10.

Also, the inferior portion 63 of the post 32 can be disposed within thecentral recess 50 of the bearing member 28. In some embodimentsrepresented in FIG. 10A, the central recess 50 of the bearing member 28is wide enough to allow the post 32 to rotate 360° about the axis A.However, in other embodiments represented in FIG. 10B, the centralrecess 50 can be shaped to abuttingly limit rotation of the post 32about the axis A (e.g., 8° or less), as shown in phantom lines. The post32 can be made out of any suitable material, such as a polymer(polyethylene).

Additionally, as shown in FIGS. 2 and 9, the washer 34 can be relativelyflat and rectangular. The washer 34 can also include projections 70(FIG. 9). The washer 34 can be fixedly coupled to the inferior portion63 of the post 32. The washer 34 can also include a central aperture 72that allows passage of the extension portion 54 of the insert 30. Also,as shown in FIG. 2, the washer 34 can be disposed between the post 32and the superior surface 46 of the bearing member 28. The washer 34 canbe made out of any suitable material, such as a metallic material(cobalt, cobalt alloy, etc.). As such, the washer 34 can inhibit wear ofthe post 32 and/or the bearing member 28 due to rotation of the post 32relative to the bearing member 28 about the axis A.

Moreover, as shown in FIGS. 2, 3A, 3B, 7, and 8, the retention member 36can be a nut with an internal thread 74. The retention member 36 can bereceived within the counterbore 66 of the post 32 to receive theextension portion 54 and threadably couple to the threaded portion 60 ofthe insert 30. Also, the retention member 36 can include aninner-projection 75 that abuts the shoulder 59 of the extension portion54 to limit movement of the retention member 36 in a direction generallyparallel to the axis A toward the post 32. The retention member 36 canalso include a groove 76 on a superior surface 78 thereof. A separatetool (not shown) can engage with the groove 76 during implantation ofthe prosthesis 10 to rotate the retention member 36 about the axis A andto threadably couple the retention member 36 to the insert 30. Theretention member 36 can be made out of any suitable material, such asmetal (cobalt, cobalt alloy, etc.). Furthermore, the counterbore 66 ofthe post 32 can have a depth D (FIG. 5) that is at least equal to alength L (FIG. 7) of the retention member 36. As such, the retentionmember 36 can be disposed entirely within the counterbore 66.

Furthermore, the knee joint prosthesis 10 can include one or moreweldments 80 (FIGS. 2, 3 a, 3 b, and 8). The weldments 80 can bond theretention member 36 to the insert 30 to inhibit the retention member 36from rotating relative to the insert 30. The weldments 80 can be made inany suitable fashion, such as via a laser welding process. As such, theweldments 80 can be made at relatively low temperatures, and thematerials of the other components of the knee joint prosthesis 10 areunlikely to be detrimentally affected by the welding process.Furthermore, the weldments 80 can be made without the use of any fillermaterial, thereby facilitating implantation of the prosthesis 10.

It will be appreciated that the fixed, threaded coupling between theretention member 36 and the insert 30 can bear axial loads to limitmovement of the post 32 axially away from the insert 30, and theweldments 80 can bear loads to limit rotation of the retention member 36about the axis A relative to the insert 30. Thus, the retention member36 can inhibit movement of the post 32 axially away from the bearingmember 28. However, the post 32 can still rotate about the axis A evenwhen the retention member 36 is fixed to the insert 30.

As shown in FIG. 1, the post 32 can be received within the opening 24 ofthe femoral component 12 of the knee joint prosthesis 10 and can camagainst inner surfaces of the opening 24 to replicate natural motion ofa knee joint. The post 32 can rotate relative to the bearing member 28and the tibial tray 26 to thereby compensate for misalignment betweenthe femoral component 12 and the tibial component 14 of the knee jointprosthesis 10. Thus, the knee joint prosthesis 10 can have a suitablerange of motion and can closely mimic natural anatomy.

In summary, the retention member 36 can retain the post 32 on the insert30 in a very robust manner. Also, the retention member 36 can facilitateassembly of the tibial component 14 of the knee joint prosthesis 10.Additionally, as discussed above, the insert 30 can be fixed to thebearing member 28 in a very robust manner. Furthermore, the knee jointprosthesis 10 can be relatively easy to implant and assemble. Also, theknee joint prosthesis 10 can be relatively easy to manufacture.

Moreover, the foregoing discussion discloses and describes merelyexemplary embodiments of the present disclosure. One skilled in the artwill readily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationsmay be made therein without departing from the spirit and scope of thedisclosure as defined in the following claims. For instance, thesequence of the blocks of the method described herein can be changedwithout departing from the scope of the present disclosure.

What is claimed is:
 1. A knee joint prosthesis for implantation toreplace a portion of a knee joint comprising: a tibial tray; a bearingmember that engages the tibial tray; an insert with a base that is fixedto the bearing member and an extension portion that extends away fromthe bearing member and the tibial tray, the extension portion having anaxis, the extension portion including a threaded portion; a post that isrotatably coupled to the extension portion, the post being rotatableabout the axis of the extension portion relative to the insert; and aretention member that threadably couples to the threaded portion of theextension portion to limit movement of the post along the axis away fromthe bearing member and the tibial tray.
 2. The knee joint prosthesis ofclaim 1, wherein the post includes a bore that receives the extensionportion to rotatably couple the post to the extension portion.
 3. Theknee joint prosthesis of claim 2, wherein the post further includes acounterbore that receives the retention member.
 4. The knee jointprosthesis of claim 3, wherein the post has a superior surface, and thecounterbore has a depth below the superior surface, the depth being atleast equal to an axial length of the retention member.
 5. The kneejoint prosthesis of claim 1, wherein the threaded portion is included onan outer surface of the extension portion, and wherein the retentionmember is a nut that receives the extension portion to threadably coupleto the threaded portion of the extension portion.
 6. The knee jointprosthesis of claim 5, wherein the nut includes a superior surface witha groove that is engageable to rotate the nut relative to the extensionportion and to threadably couple the nut to the extension portion. 7.The knee joint prosthesis of claim 1, further comprising a washer thatis disposed between the post and the bearing member, the washerinhibiting wear due to rotation of the post relative to the bearingmember.
 8. The knee joint prosthesis of claim 1, wherein the bearingmember includes a polymeric material, wherein the post includes apolymeric material, wherein the tibial tray includes a metallicmaterial, wherein the insert includes a metallic material, and whereinthe retention member includes a metallic material.
 9. The knee jointprosthesis of claim 1, wherein the base of the insert member includes aninferior terminal end that is disposed superior to a tray engagingsurface of the bearing member.
 10. The knee joint prosthesis of claim 1,further comprising a femoral component with an inferior portion havingan opening, the inferior portion being coupled for articulation on thebearing member, the post being moveably received within the opening. 11.A knee joint prosthesis for implantation to replace a portion of a kneejoint comprising: a tibial tray; a bearing member that engages thetibial tray; an insert with a base that is fixed to the bearing memberand an extension portion that extends away from the bearing member andthe tibial tray, the extension portion having an axis, wherein the baseis disc-shaped and concentric about the axis; a post that is rotatablycoupled to the extension portion, the post being rotatable about theaxis of the extension portion relative to the insert; and a retentionmember that couples to the extension portion to limit movement of thepost along the axis away from the bearing member and the tibial tray.12. The knee joint prosthesis of claim 11, further comprising a weldmentthat bonds the retention member to the insert.
 13. The knee jointprosthesis of claim 11 wherein the post includes a bore that receivesthe extension portion to rotatably couple the post to the extensionportion.
 14. The knee joint prosthesis of claim 13 wherein the postfurther includes a counterbore that receives the retention member. 15.The knee joint prosthesis of claim 11 wherein the extension portionfurther comprises a threaded portion, and wherein the retention memberis a nut that receives the extension portion to threadably couple to thethreaded portion of the extension portion.
 16. The knee joint prosthesisof claim 11 wherein the insert and the base are monolithic.
 17. A kneejoint prosthesis for implantation to replace a portion of a knee jointcomprising: a tibial tray; a bearing member that engages the tibialtray; an insert with a base that is fixed to the bearing member and anextension portion that extends away from the bearing member and thetibial tray, the extension portion having an axis; a post that isrotatably coupled to the extension portion, the post defining acounterbore and being rotatable about the axis of the extension portionrelative to the insert; and a retention member that is received into thecounterbore and that couples to the insert to limit movement of the postalong the axis away from the bearing member and the tibial tray.
 18. Theknee joint prosthesis of claim 17 wherein the retention member includesan inner projection that abuts a shoulder of the extension portion. 19.The knee joint prosthesis of claim 17, further comprising a washer thatis disposed between the post and the bearing member, the washerinhibiting wear due to rotation of the post relative to the bearingmember.